November 2002 Interface Design for Oxidation-Resistant Ceramic Composites the change in chemistry, and probably elastic properties. may environmental resistance have been studied, Periodic matrix introduce some ambiguity in interpretation of results cracks, nonlinear load displacement, and hysteresis during unload- Porous-matrix CMCs without fiber coatings can have attractive load cycles have been observed, from which debond properties via distributed damage mechanisms, because cracks and the average friction( T)have been estimated. However. full deflect around fibers without need for a coating (see Section confidence in validity of the results for property prediction in a full V().Matrix pore volume fractions at which significant tough CMC has not been established. ening is observed range from >30% to 15%..Hence, porous Oxide/oxide microcomposites have been fabricated and tested matrices complicate evaluation of fiber coatings, because the to evaluate the effectiveness of monazite(LaPO)and hibonite hening Therefore, better understanding of damage mechanisms in porous- composites. Using sapphire monofilaments in an Al,O, matrix matrix composites may be necessary for complete understanding as the control composites, the fractography and fracture strengths of damage mechanisms in coated-fiber composites with imper were compared. For interlayer thicknesses of 0.3-0.5 um, both fectly densified matricesusually the case. interlayers showed evidence of crack deflection; however debond lengths in hibonite-coated specimens were limited to just a small (4) Micro- and Mini-CMCs fraction of the fiber diameter. Monazite-coated specimens showed Use of micro or mini-CMCs for more-rapid evaluation has nultiple matrix cracks and extensive debonding at the coating/ received increasing attention. 00. Io A micro-CMC is defined as a is defined as a matrix interface. In both cases, the load-displacement curves were cylindrical matrix reinforced with one fiber, while a mini-CMC almost linear to failure: therefore there was no unload-reload uses one or multiple fiber tows(200-3000 fibers/tow and up to hysteresis from which to measure interfacial friction. Failure four tows). The mechanical behavior of a mini-CMC is more strength was the only measurable mechanical parameter. The difficult to interpret, but it includes the statistical nature of fiber extent of nonlinearity in tension of specimens of any typ fracture and is more representative of a real composite. These high fiber modulus, straight fibers, and low matrix volume fraction micro- and mini-CMCs are easier to fabricate than full CMCs, and relaxed sintering constraints on matrix densification can allow hypothesis that, even if the coating and matrix volume fraction is denser matrices to be more easily made. 2103 Most such tests very low, there is severe degradation in apparent fiber strength if have been limited to carbon and BN fiber/matrix interfaces and there is no mechanism to deflect cracks. The matrix and coating mostly CVI-SiC matrices. Effects of fiber surface treatments or crack at relatively low strain, and, unless the crack deflects, it acts coating procedures on interface properties and evaluation of as a large flaw in the fiber. In this experiment, composite strengths Sapphire Matrix Hibonite Debonded Surface (Matrix dislodged Alumina CMC-Control CMC-Control 1.18 GPa 0F 1.18GPa m=52 Fiber-l45Q°chh 225GP F Tiber/Mon A FiberHibonite CMC- Hibonite 1450c2h -4 1.18 GPa 1. 84GPa 日 1.28 GPa m=10.3 0.500 11.5 -1-0.50 0.5 1.5 Ln Stress, GPa I Ln Stress, GPa Fig. 10. Single-filament forced/Al, O, matrix microcomposites tested in tension: (a) cracks deflect within the hibonite intertace but a fiber diameter, because of the roughness;(b) present at the monazite/matrix interface are revealed by matrix regions that fell and (d)microcomposites with coatings have no coatings, but the Weibu he same mean strengths as the control composites with about the same as the coated fibers, Results he matrix is not sufficiently dense for evaluation of